The present invention relates, in general, to integrated circuits and, more particularly, to programmable integrated circuits.
Some precision applications such as filters, amplifiers, delay circuits, etc. use circuits requiring a constant current. For example, a differential stage of an amplifier is typically coupled to a current mirror stage that requires a constant current. Ideally, under quiescent conditions each device of the current mirror stage conducts equal currents. An offset voltage would produce unequal currents in the current mirror stage. One technique commonly used to reduce an offset voltage of a differential input stage is to change the currents conducted by the current mirror stage under quiescent conditions to be equal. Accordingly, current sources have been used to steer supply currents to, or away from, the differential transistor pair in such a way that the sum of the operating currents for the input transistor pair is largely constant.
An offset voltage of an amplifier results from device mismatches of an input differential transistor pair due to wafer process variations in fabricating the devices. An offset voltage is amplified by an amplifier when placed in a gain configuration, producing a DC voltage at an output of the amplifier that reduces performance of the differential amplifier. One consequence of voltage offset in a differential input stage is to reduce the dynamic output range for low voltage amplifiers.
Other applications such as delay circuits or filters use analog integrated circuits having resistor-capacitor (RC) components tuned to provide a desired time delay. Typically, the resistor used in the time delay RC component is implemented using a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) operated in the triode or linear resistance region. This is accomplished by applying an analog voltage to the gate of the MOSFET. Typically, time constants of the associated circuitry do not match because they are dependent on the physical layout of the filter RC components as well as the processes used to manufacture the filter and time delay components.
Accordingly, it would be advantageous to have a device that conducts a constant current. In addition, it is desirable that the device be programmable to account for temperature and voltage changes that affect the constant current.